A positive-electrode active material containing a compound that has a crystal structure belonging to the space group fm-3M and is represented by the composition formula (1):
LixMeyOαFβ (1)
wherein Me denotes one or two or more elements selected from the group consisting of Mn, Co, Ni, Fe, and Al, and the following conditions are satisfied.
1.3≤x≤2.2,
0.8≤y≤1.3,
1≤α≤2.93,
0.07≤β≤2.
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1. A positive-electrode active material comprising a compound that has a crystal structure belonging to a space group fm-3M and is represented by the composition formula (1):
LixMeyOαFβ (1) wherein Me denotes one or two or more elements selected from the group consisting of Mn, Co, Ni, Fe, and Al, and the following conditions are satisfied,
1.3≤x≤2.2,
0.8≤y≤1.3,
1≤α≤2.93,
0.07≤β≤2, and
1.5≤(4y−β)/α≤2.2.
2. The positive-electrode active material according to
3. The positive-electrode active material according to
4. The positive-electrode active material according to
5. The positive-electrode active material according to
6. The positive-electrode active material according to
7. The positive-electrode active material according to
8. The positive-electrode active material according to
9. The positive-electrode active material according to
10. A battery comprising:
a positive electrode containing the positive-electrode active material according to
a negative electrode; and
an electrolyte.
11. The battery according to
12. The battery according to
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This application is a Continuation of U.S. patent application Ser. No. 15/811,685, filed on Nov. 14, 2017, now U.S. Pat. No. 10,818,910, which is a Continuation of International Patent Application No. PCT/JP2016/003215, filed on Jul. 6, 2016, which in turn claims the benefit of Japanese Application No. 2015-145485, filed on Jul. 23, 2015, the entire disclosures of which Applications are incorporated by reference herein.
The present disclosure relates to a positive-electrode active material for use in batteries and to a battery.
Japanese Unexamined Patent Application Publication No. 7-037617 discloses a positive-electrode active material that has a crystal structure belonging to the space group R3-M and is represented by the formula LiwMxOyXz (M denotes Co, Ni, Mn, V, Fe, or Ti, X denotes at least one halogen element, 0.2≤w≤2.5, 0.8≤x≤1.25, 1≤y≤2, 0<z≤1).
In the related art, there is a demand for high-capacity batteries.
In one general aspect, the techniques disclosed here feature a positive-electrode active material containing a compound that has a crystal structure belonging to the space group FM-3M and that is represented by the composition formula (1).
LixMeyOαFβ (1)
wherein Me denotes one or two or more elements selected from the group consisting of Mn, Co, Ni, Fe, and Al, and the following conditions are satisfied.
1.3≤x≤2.2
0.8≤y≤1.3
1≤α≤2.93
0.07≤β≤2
The present disclosure can provide a high-capacity battery.
Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages.
Embodiments of the present disclosure will be described below.
A positive-electrode active material of a first embodiment contains a compound that has a crystal structure belonging to the space group FM-3M and that is represented by the composition formula (1).
LixMeyOαFβ (1)
wherein Me denotes at least one selected from the group consisting of Mn, Co, Ni, Fe, and Al (that is, one or two or more elements selected from the group).
In the positive-electrode active material of the first embodiment, the compound satisfies the following conditions in the composition formula (1).
1.3≤x≤2.2
0.8≤y≤1.3
1≤α≤2.93
0.07≤β≤2
Such an embodiment can provide a high-capacity battery.
For example, a lithium-ion battery containing a positive-electrode active material containing the compound has an oxidation-reduction potential (vs. Li/Li+) of approximately 3.3 V and a capacity of approximately 220 mAh/g or more.
In the composition formula (1), x of less than 1.3 indicates that the compound has a decreased amount of available Li and insufficient capacity.
In the composition formula (1), x of more than 2.2 (or y of less than 0.8) indicates decreased utilization of an oxidation-reduction reaction of an available transition metal in the compound. This results in increased utilization of an oxidation-reduction reaction of oxygen. This destabilizes the crystal structure and results in insufficient capacity.
In the composition formula (1), α of less than 1 (or β of more than 2) indicates that the effects of highly electronegative F on the compound increase. This results in decreased electron conductivity and insufficient capacity.
In the composition formula (1), α of more than 2.93 (or β of less than 0.07) indicates that the effects of highly electronegative F on the compound decrease. This decreases cation-anion interaction. This destabilizes the structure when Li is desorbed and therefore results in insufficient capacity.
In the positive-electrode active material of the first embodiment, a compound represented by the composition formula (1) has a crystal structure belonging to the space group FM-3M (rock-salt-type crystal structure).
In the composition formula (1), the ratio of Li to Me is represented by {Lix/Mey}.
By way of example, the case where 1.7≤x≤2.2 and 0.8≤y≤1.3 is described below.
In this case, the ratio of Li to Me is theoretically 1.3≤{Lix/Mey}≤2.75 and is more than 1.
The number of Li atoms per Me atom is larger than that for a known positive-electrode active material, for example, LiMnO2.
In a compound represented by the composition formula (1), Li and Me are probably located at the same site.
Thus, a compound represented by the composition formula (1) can intercalate and deintercalate more Li per Me atom than a known positive-electrode active material, for example, LiMnO2.
Thus, the positive-electrode active material of the first embodiment is suitable for high-capacity lithium-ion batteries.
Upon abstraction of much Li, a layered structure specified by the space group R3-M cannot hold the layers and disintegrates.
By contrast, a rock-salt-type crystal structure specified by the space group FM-3M, such as a compound according to the present disclosure, can stably maintain the structure without disintegration, even after abstraction of much Li. Thus, a high-capacity battery can be provided.
The compound may constitute 90% or more by weight of the positive-electrode active material of the first embodiment.
Such an embodiment can provide a higher-capacity battery.
In addition to the compound, the positive-electrode active material of the first embodiment may contain incidental impurities, or starting materials for the synthesis of the compound, by-products, and degradation products.
The compound may constitute 100% by weight of the positive-electrode active material of the first embodiment except incidental impurities.
Such an embodiment can provide a higher-capacity battery.
In the positive-electrode active material of the first embodiment, the compound may satisfy 0.8≤(x+y)/(α+β)≤1 in the composition formula (1).
Such an embodiment can provide a higher-capacity battery.
In the positive-electrode active material of the first embodiment, the compound may satisfy 2.5/3≤(x+y)/(α+β)≤2.6/3 in the composition formula (1).
Such an embodiment can provide a higher-capacity battery.
In the positive-electrode active material of the first embodiment, the compound may satisfy 2.83<{2α+β−(3−y)}/y in the composition formula (1).
Such an embodiment can provide a higher-capacity battery.
In the positive-electrode active material of the first embodiment, Me in the composition formula (1) may be one element selected from Mn, Co, Ni, and Fe, a solid solution composed of Ni, Co, and Mn, a solid solution composed of Ni, Co, and Al, a solid solution composed of Mn and Co, or a solid solution composed of Mn and Ni.
Such an embodiment can provide a higher-capacity battery.
In the positive-electrode active material of the first embodiment, the compound may satisfy 1.79≤x≤2.18 in the composition formula (1).
Such an embodiment can provide a higher-capacity battery.
In the positive-electrode active material of the first embodiment, the compound may satisfy 1.89≤x≤2 in the composition formula (1).
Such an embodiment can provide a higher-capacity battery.
In the positive-electrode active material of the first embodiment, the compound may satisfy 0.5≤β in the composition formula (1).
Such an embodiment can provide a battery with better cycle characteristics. In the composition formula (1), β of less than 0.5 (that is, a small amount of fluorine) indicates that more oxygen in the compound is involved in a redox reaction. Thus, the structure is likely to become unstable due to desorption of oxygen. This tends to result in poor cycle characteristics.
In the positive-electrode active material of the first embodiment, the compound may satisfy 0.79≤β≤1 in the composition formula (1).
Such an embodiment can provide a higher-capacity battery.
<Method for Producing Compound>
An exemplary method for producing the compound of the positive-electrode active material of the first embodiment will be described below.
A compound represented by the composition formula (1) can be produced by the method described below, for example.
A raw material containing Li, a raw material containing F, and a raw material containing Me are prepared. Examples of the raw material containing Li include oxides, such as Li2O and Li2O2, salts, such as LiF, Li2CO3, and LiOH, and lithium composite transition metal oxides, such as LiMeO2 and LiMe2O4. Examples of the raw material containing F include LiF and transition metal fluorides. Examples of the raw material containing Me include oxides with various oxidation states, such as Me2O3, salts, such as MeCO3 and MeNO3, hydroxides, such as Me(OH)2 and MeOOH, and lithium composite transition metal oxides, such as LiMeO2 and LiMe2O4. In the case that Me is Mn, examples of the raw material containing Mn include manganese oxides with various oxidation states, such as Mn2O3, salts, such as MnCO3 and MnNO3, hydroxides, such as Mn(OH)2 and MnOOH, and lithium composite transition metal oxides, such as LiMnO2 and LiMn2O4.
These raw materials are weighed at the mole ratio of the composition formula (1).
The variables “x, y, α, and β” in the composition formula (1) can be altered in the ranges described for the composition formula (1).
The weighed raw materials are mixed, for example, by a dry process or a wet process and are allowed to react mechanochemically for 10 hours or more to produce a compound represented by the composition formula (1). For example, a mixing apparatus, such as a ball mill, may be used.
The raw materials and the conditions for mixing a mixture of the raw materials can be adjusted to produce a compound substantially represented by the composition formula (1).
The use of a lithium transition metal composite oxide as a precursor can decrease the energy for mixing elements. Thus, a compound represented by the composition formula (1) can be produced with higher purity.
The composition of a compound represented by the composition formula (1) thus produced can be determined by ICP spectroscopy and an inert gas fusion-infrared absorption method, for example.
A compound represented by the composition formula (1) can be identified by determining the space group of the crystal structure by powder X-ray analysis.
Thus, a method for producing a positive-electrode active material according to one aspect of the first embodiment includes (a) a step of preparing the raw materials and (b) a step of mechanochemically reacting the raw materials to produce the positive-electrode active material.
The step (a) may include a step of mixing a raw material containing Li and F and a raw material containing Me at a Li/Me mole ratio in the range of 1.31 to 2.33 to prepare a raw material mixture.
The step (a) may include a step of producing a raw material, a lithium transition metal composite oxide, by a known method.
The step (a) may include a step of mixing the raw materials at a Li/Me mole ratio in the range of 1.7 to 2.0 to prepare a raw material mixture.
The step (b) may include a step of mechanochemically reacting the raw materials in a ball mill.
Thus, a compound represented by the composition formula (1) may be synthesized by mechanochemically reacting a precursor (for example, LiF, Li2O, a transition metal oxide, a lithium composite transition metal, etc.) in a planetary ball mill.
The mixing ratio of the precursor can be adjusted to introduce more Li atoms.
When the precursor is reacted by a solid phase method, the precursor is decomposed into more stable compounds.
Thus, a compound that has a crystal structure belonging to the space group FM-3M and is represented by the composition formula (1) cannot be produced by a method for reacting the precursor by a solid phase method.
A second embodiment will be described below. The contents described in the first embodiment are appropriately omitted to avoid overlap.
A battery of the second embodiment includes a positive electrode containing the positive-electrode active material of the first embodiment, a negative electrode, and an electrolyte.
Such an embodiment can provide a high-capacity battery.
More specifically, as described in the first embodiment, the positive-electrode active material contains many Li atoms per Me atom. Thus, a high-capacity battery can be provided.
The battery of the second embodiment may be a lithium-ion secondary battery or a non-aqueous electrolyte secondary battery, for example.
In the battery of the second embodiment, the positive electrode may have a positive-electrode active material layer. The positive-electrode active material layer may contain the positive-electrode active material of the first embodiment (the compound of the first embodiment) as a main component (that is, 50% or more by weight (50 wt % or more) of the positive-electrode active material layer).
Such an embodiment can provide a higher-capacity battery.
Alternatively, the positive-electrode active material layer in the battery of the second embodiment may contain the positive-electrode active material of the first embodiment (the compound of the first embodiment) constituting 70% or more by weight (70 wt % or more) of the positive-electrode active material layer.
Such an embodiment can provide a higher-capacity battery.
Alternatively, the positive-electrode active material layer in the battery of the second embodiment may contain the positive-electrode active material of the first embodiment (the compound of the first embodiment) constituting 90% or more by weight of the positive-electrode active material layer (90 wt % or more).
Such an embodiment can provide a higher-capacity battery.
In the battery of the second embodiment, for example, the negative electrode may contain a negative-electrode active material that can adsorb and desorb lithium (for example, a negative-electrode active material with lithium adsorption and desorption characteristics).
In the battery of the second embodiment, for example, the electrolyte may be a non-aqueous electrolyte (for example, a non-aqueous electrolyte solution).
As illustrated in
The separator 14 is disposed between the positive electrode 21 and the negative electrode 22.
The positive electrode 21, the negative electrode 22, and the separator 14 are impregnated with a non-aqueous electrolyte (for example, a non-aqueous electrolyte solution).
The positive electrode 21, the negative electrode 22, and the separator 14 constitute an electrode group.
The electrode group is housed in the case 11.
The case 11 is sealed with the gasket 18 and the sealing plate 15.
The positive electrode 21 includes a positive-electrode current collector 12 and a positive-electrode active material layer 13 disposed on the positive-electrode current collector 12.
The positive-electrode current collector 12 is formed of a metallic material (aluminum, stainless steel, aluminum alloy, etc.), for example.
The positive-electrode current collector 12 may be omitted, and the case 11 may be used as a positive-electrode current collector.
The positive-electrode active material layer 13 contains the positive-electrode active material of the first embodiment.
If necessary, the positive-electrode active material layer 13 may contain an additive agent (electrically conductive agent, ionic conduction aid, binder, etc.).
The negative electrode 22 includes a negative-electrode current collector 16 and a negative-electrode active material layer 17 disposed on the negative-electrode current collector 16.
The negative-electrode current collector 16 is formed of a metallic material (aluminum, stainless steel, aluminum alloy, etc.), for example.
The negative-electrode current collector 16 may be omitted, and the sealing plate 15 may be used as a negative-electrode current collector.
The negative-electrode active material layer 17 contains a negative-electrode active material.
If necessary, the negative-electrode active material layer 17 may contain an additive agent (electrically conductive agent, ionic conduction aid, binder, etc.).
The negative-electrode active material may be a metallic material, carbon material, oxide, nitride, tin compound, or silicon compound.
The metallic material may be a single metal or an alloy. Examples of the metallic material include lithium metals and lithium alloys.
Examples of the carbon material include natural graphite, coke, carbon under graphitization, carbon fiber, spherical carbon, artificial graphite, and amorphous carbon.
From the perspective of capacity density, silicon (Si), tin (Sn), silicon compounds, and tin compounds can be suitably used. Silicon compounds and tin compounds may be alloys and solid solutions.
Examples of the silicon compounds include SiOx (wherein 0.05<x<1.95). Compounds (alloys and solid solutions) produced by substituting another element for part of silicon of SiOx may also be used. The other element may be at least one selected from the group consisting of boron, magnesium, nickel, titanium, molybdenum, cobalt, calcium, chromium, copper, iron, manganese, niobium, tantalum, vanadium, tungsten, zinc, carbon, nitrogen, and tin.
Examples of the tin compound include Ni2Sn4, Mg2Sn, SnOx (wherein 0<x<2), SnO2, and SnSiO3. One tin compound selected from these compounds may be used alone. Alternatively, two or more tin compounds selected from these compounds may be used in combination.
The negative-electrode active material may have any shape. The negative-electrode active material may have a known shape (particulate, fibrous, etc.).
The negative-electrode active material layer 17 may be filled with (adsorb) lithium by any method. More specifically, the method may be (a) a method for depositing lithium on the negative-electrode active material layer 17 by a gas phase method, such as a vacuum evaporation method, or (b) a method for heating a lithium metal foil in contact with the negative-electrode active material layer 17. In these methods, lithium can be diffused into the negative-electrode active material layer 17 by heat. Alternatively, lithium may be electrochemically adsorbed on the negative-electrode active material layer 17. More specifically, a battery is assembled from the negative electrode 22 free of lithium and a lithium metal foil (positive electrode). Subsequently, the battery is charged to adsorb lithium on the negative electrode 22.
Examples of the binder for the positive electrode 21 and the negative electrode 22 include poly(vinylidene difluoride), polytetrafluoroethylene, polyethylene, polypropylene, aramid resin, polyamide, polyimide, polyamideimide, polyacrylonitrile, poly(acrylic acid), poly(methyl acrylate), poly(ethyl acrylate), poly(hexyl acrylate), poly(methacrylic acid), poly(methyl methacrylate), poly(ethyl methacrylate), poly(hexyl methacrylate), poly(vinyl acetate), polyvinylpyrrolidone, polyether, polyethersulfone, hexafluoropolypropylene, styrene-butadiene rubber, and carboxymethylcellulose. Other examples of the binder include copolymers of two or more materials selected from the group consisting of tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethyl vinyl ether, acrylic acid, and hexadiene. The binder may also be a mixture of two or more materials selected from these materials.
Examples of the electrically conductive agent for the positive electrode 21 and the negative electrode 22 include graphite, carbon black, electrically conductive fiber, graphite fluoride, metal powders, electrically conductive whiskers, electrically conductive metal oxides, and electrically conductive organic materials. Examples of the graphite include natural graphite and artificial graphite. Examples of the carbon black include acetylene black, ketjen black (registered trademark), channel black, furnace black, lampblack, and thermal black. Examples of the metal powders include aluminum powders. Examples of the electrically conductive whiskers include zinc oxide whiskers and potassium titanate whiskers. Examples of the electrically conductive metal oxides include titanium oxide. Examples of the electrically conductive organic materials include phenylene derivatives.
The separator 14 may be formed of a material that has high ion permeability and sufficient mechanical strength. Examples of such a material include microporous thin films, woven fabrics, and nonwoven fabrics. More specifically, it is desirable that the separator 14 be formed of a polyolefin, such as polypropylene or polyethylene. The separator 14 formed of a polyolefin has not only good durability but also a shutdown function in case of excessive heating. The separator 14 has a thickness in the range of 10 to 300 μm (or 10 to 40 μm), for example. The separator 14 may be a monolayer film formed of one material. Alternatively, the separator 14 may be a composite film (or multilayer film) formed of two or more materials. The separator 14 has a porosity in the range of 30% to 70% (or 35% to 60%), for example. The term “porosity”, as used herein, refers to the volume ratio of pores to the separator 14. The “porosity” is measured by a mercury intrusion method, for example.
The non-aqueous electrolyte solution contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent.
Examples of the non-aqueous solvent include cyclic carbonate solvents, chain carbonate solvents, cyclic ether solvents, chain ether solvents, cyclic ester solvents, chain ester solvents, and fluorinated solvents.
Examples of the cyclic carbonate solvents include ethylene carbonate, propylene carbonate, and butylene carbonate.
Examples of the chain carbonate solvents include dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.
Examples of the cyclic ether solvents include tetrahydrofuran, 1,4-dioxane, and 1,3-dioxolane.
Examples of the chain ether solvents include 1,2-dimethoxyethane and 1,2-diethoxyethane.
Examples of the cyclic ester solvent include γ-butyrolactone.
Examples of the chain ester solvents include methyl acetate.
Examples of the fluorinated solvents include fluoroethylene carbonate, methyl fluoropropionate, fluorobenzene, fluoroethyl methyl carbonate, and fluorodimethylene carbonate.
The non-aqueous solvent may be one non-aqueous solvent selected from these. Alternatively, the non-aqueous solvent may be a combination of two or more non-aqueous solvents selected from these.
The non-aqueous electrolyte solution may contain at least one fluorinated solvent selected from the group consisting of fluoroethylene carbonate, methyl fluoropropionate, fluorobenzene, fluoroethyl methyl carbonate, and fluorodimethylene carbonate.
These fluorinated solvents in the non-aqueous electrolyte solution improve the oxidation resistance of the non-aqueous electrolyte solution.
Consequently, even when the battery 10 is charged at a high voltage, the battery 10 can operate stably.
Examples of the lithium salt include LiPF6, LiBF4, LiSbF6, LiAsF6, LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), and LiC(SO2CF3)3. The lithium salt may be one lithium salt selected from these. Alternatively, the lithium salt may be a mixture of two or more lithium salts selected from these. The concentration of the lithium salt ranges from 0.5 to 2 mol/liter, for example.
The battery of the second embodiment may be of various types, such as coin type, cylindrical type, square or rectangular type, sheet type, button type, flat type, or layered type.
A third embodiment will be described below. The contents described in the first or second embodiment are appropriately omitted to avoid overlap.
A positive-electrode active material of the third embodiment contains a compound represented by the composition formula (1).
In the composition formula (1), Me denotes at least one selected from the group consisting of Mn, Co, Ni, Fe, and Al (that is, one or two or more elements selected from the group).
In the positive-electrode active material of the third embodiment, the compound satisfies the following conditions in the composition formula (1).
1.3≤x≤2.2
0.8≤y≤1.3
1≤α≤2.93
0.07≤β≤2
Such an embodiment can provide a high-capacity battery.
In the positive-electrode active material of the third embodiment, the compound may satisfy 0.8≤(x+y)/(α+β)≤1 in the composition formula (1).
Such an embodiment can provide a higher-capacity battery.
In the positive-electrode active material of the third embodiment, the compound may satisfy 2.5/3≤(x+y)/(α+β)≤2.6/3 in the composition formula (1).
Such an embodiment can provide a higher-capacity battery.
In the positive-electrode active material of the third embodiment, the compound may satisfy 2.83<{2α+β−(3−y)}/y in the composition formula (1).
Such an embodiment can provide a higher-capacity battery.
In the positive-electrode active material of the third embodiment, Me in the composition formula (1) may be one element selected from Mn, Co, Ni, and Fe, a solid solution composed of Ni, Co, and Mn, a solid solution composed of Ni, Co, and Al, a solid solution composed of Mn and Co, or a solid solution composed of Mn and Ni.
Such an embodiment can provide a higher-capacity battery.
In the positive-electrode active material of the third embodiment, the compound may satisfy 1.79≤x≤2.18 in the composition formula (1).
Such an embodiment can provide a higher-capacity battery.
In the positive-electrode active material of the third embodiment, the compound may satisfy 1.89≤x≤2 in the composition formula (1).
Such an embodiment can provide a higher-capacity battery.
In the positive-electrode active material of the third embodiment, the compound may satisfy 0.5≤β in the composition formula (1).
Such an embodiment can provide a battery with better cycle characteristics. In the composition formula (1), β of less than 0.5 (that is, a small amount of fluorine) indicates that more oxygen in the compound is involved in a redox reaction. Thus, the structure is likely to become unstable due to desorption of oxygen. This tends to result in poor cycle characteristics.
In the positive-electrode active material of the third embodiment, the compound may satisfy 0.79≤β≤1 in the composition formula (1).
Such an embodiment can provide a higher-capacity battery.
A positive electrode containing the positive-electrode active material of the third embodiment, a negative electrode, and an electrolyte may constitute a battery.
Such an embodiment can provide a high-capacity battery.
[Production of Positive-Electrode Active Material]
LiF and LiMnO2 were weighed at a mole ratio of LiF/LiMnO2=1.0/1.0.
The raw materials, together with a proper amount of ϕ3-mm zirconia balls, were put in a 45-cc zirconia container, which was then sealed in an argon glove box.
It was removed from the argon glove box and was treated in a planetary ball mill at 600 rpm for 30 hours.
The resulting compound was subjected to X-ray powder diffraction measurement.
The space group of the compound was FM-3M.
The composition of the compound was determined by ICP spectroscopy and inert gas fusion-infrared absorption.
The composition of the compound was Li2MnO2F.
[Production of Battery]
70 parts by mass of the compound, 20 parts by mass of an electrically conductive agent, 10 parts by mass of poly(vinylidene difluoride) (PVDF), and a proper amount of 2-methylpyrrolidone (NMP) were mixed to prepare a positive-electrode mixture slurry.
The positive-electrode mixture slurry was applied to one side of a positive-electrode current collector formed of aluminum foil 20 μm in thickness.
The positive-electrode mixture slurry was dried and rolled to form a positive-electrode plate with a positive-electrode active material layer. The positive-electrode plate had a thickness of 60 μm.
A circular positive electrode 12.5 mm in diameter was punched out from the positive-electrode plate.
A circular negative electrode 14.0 mm in diameter was punched out from lithium metal foil 300 μm in thickness.
Fluoroethylene carbonate (FEC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1:1:6 to prepare a non-aqueous solvent.
LiPF6 was dissolved at a concentration of 1.0 mol/liter in the non-aqueous solvent to prepare a non-aqueous electrolyte solution.
A separator (manufactured by Celgard, LLC., product number 2320, 25 μm in thickness) was impregnated with the non-aqueous electrolyte solution.
Celgard (registered trademark) 2320 is a 3-layer separator composed of a polypropylene layer, a polyethylene layer, and a polypropylene layer.
A CR2032 coin-type battery was assembled from the positive electrode, the negative electrode, and the separator in a dry box maintained at a dew point of −50° C.
The Li/Me ratio and the O/F ratio were changed from those described in Example 1.
Table 1 lists the precursors for the production of positive-electrode active materials of Examples 2 to 18 and the compositions of the positive-electrode active materials thus synthesized.
Except for these, the positive-electrode active materials of Examples 2 to 18 were synthesized in the same manner as in Example 1.
The precursors of Examples 2 to 18 were weighed at the stoichiometric ratio and were mixed in the same manner as in Example 1. For example, in Example 2, the precursors were weighed at a mole ratio of Li2O/LiF/Mn2O3=1/2/1.
Coin-type batteries of Examples 2 to 18 were produced from the positive-electrode active materials of Examples 2 to 18 in the same manner as in Example 1.
LiF and LiMnO2 were weighed at a mole ratio of LiF/LiMnO2=1.0/1.0.
The raw material mixture was fired in an inert gas atmosphere at 800° C. Thus, particles of a lithium manganese composite compound (Li2MnO2F) were formed.
The space group of the lithium manganese composite compound particles was R3-M.
A coin-type battery of Comparative Example 1 was produced in the same manner as in Example 1 by using the lithium manganese composite compound particles as a positive-electrode active material.
Lithium cobalt oxide (LiCoO2) was produced by a known method.
The space group of the lithium cobalt oxide was R3-M.
A coin-type battery of Comparative Example 2 was produced in the same manner as in Example 1 by using the lithium cobalt oxide as a positive-electrode active material.
The precursors for the production of a positive-electrode active material, the Li/Me ratio, and the O/F ratio were changed from those described in Comparative Example 1.
Table 1 lists the precursors for the production of positive-electrode active materials of Comparative Examples 3 and 4 and the compositions of the positive-electrode active materials thus synthesized.
Except for these, the positive-electrode active materials of Comparative Examples 3 and 4 were synthesized in the same manner as in Comparative Example 1.
The precursors of Comparative Examples 3 and 4 were weighed at the stoichiometric ratio and were mixed in the same manner as in Example 1.
The space group of the lithium manganese composite compound particles thus produced was R3-M.
Coin-type batteries of Comparative Examples 3 and 4 were produced in the same manner as in Example 1 by using the lithium manganese composite compound particles as a positive-electrode active material.
<Evaluation of Battery>
The electric current density on the positive electrode was set at 0.005 mA/cm2, and the battery of Example 1 was charged to a voltage of 5.2 V.
Subsequently, the discharge cut-off voltage was set at 1.5 V, and the battery of Example 1 was discharged at an electric current density of 0.05 mA/cm2.
The initial discharge capacity was 343 mAh/g.
The electric current density on the positive electrode was set at 0.05 mA/cm2, and the battery of Comparative Example 1 was charged to a voltage of 5.2 V.
Subsequently, the discharge cut-off voltage was set at 1.5 V, and the battery of Comparative Example 1 was discharged at an electric current density of 0.005 mA/cm2.
The initial discharge capacity of the battery of Comparative Example 1 was 130 mAh/g.
The electric current density on the positive electrode was set at 0.05 mA/cm2, and the battery of Comparative Example 2 was charged to a voltage of 4.3 V.
Subsequently, the discharge cut-off voltage was set at 2.5 V, and the battery of Comparative Example 2 was discharged at an electric current density of 0.005 mA/cm2.
The initial discharge capacity of the battery of Comparative Example 2 was 145 mAh/g.
The capacities of the coin-type batteries of Comparative Examples 3 and 4 were measured in the same manner as in Comparative Example 2.
The capacities of the coin-type batteries of Examples 2 to 18 were measured in the same manner as in Example 1. Table 1 shows the results.
Table 1 lists “(x+y)/(α+β)”, which indicates deficiency, “{2α+β−(3−y)}/y”, which indicates the Me valence, and “(4y −β)/α”, which indicates the valence of oxygen after change, for each sample.
TABLE 1
initial
discharge
Space
(x + y)/
{2α + β −
(4y −
capacity
Sample
Precursor
Composition
group
(α + β)
(3 − y)}/y
β)/α
mAh/g
Example 1
LiF—LiMnO2
Li2MnO2F
FM-3M
1
3.00
1.50
343
Example 2
Li2O—LiF—Mn2O3
Li2MnO2F
FM-3M
1
3.00
1.50
323
Example 3
Li2O—LiF—Mn2O3
Li2.18Mn0.82O1.6F1.4
FM-3M
1
2.95
1.18
233
Example 4
Li2O—LiF—Mn2O3
Li1.34Mn1.06O2.12F0.88
FM-3M
1
3.00
1.58
315
Example 5
Li2O—LiF—Mn2O3
Li1.89Mn1.11O2.21F0.79
FM-3M
1
2.99
1.65
323
Example 6
Li2O—LiF—Mn2O3
Li1.79Mn1.21O2.42F0.58
FM-3M
1
3.00
1.76
290
Example 7
Li2O—LiF—MnO
Li2MnOF2
FM-3M
1
2.00
2.00
244
Example 8
Li2O—LiF—MnO—Mn2O3
Li2MnO1.5F1.5
FM-3M
1
2.50
1.67
273
Example 9
Li2O—LiF—MnO2—Mn2O3
Li2MnO2.5F0.5
FM-3M
1
3.50
1.40
256
Example 10
Li2O—LiF—Fe2O3
Li2FeO2F
FM-3M
1
3.00
1.50
220
Example 11
LiF—LiCoO2
Li2CoO2F
FM-3M
1
3.00
1.50
249
Example 12
LIF—LiNiO2
Li2NiO2F
FM-3M
1
3.00
1.50
282
Example 13
LiF—LiNi0.33Co0.33Mn0.33O2
Li2Ni0.33Co0.33Mn0.33O2F
FM-3M
1
3.00
1.50
274
Example 14
LiF—LiNi0.85Co0.01Al0.05O2
Li2Ni0.85Co0.1Al0.05O2F
FM-3M
1
3.00
1.50
285
Example 15
Li2O—LiF—MnO—Mn2O3
Li1.8Mn1.2O2F
FM-3M
1
2.67
1.90
282
Example 16
Li2O—LiF—MnO2—Mn2O3
Li1.8Mn1.2O2.5F0.5
FM-3M
1
3.08
1.72
326
Example 17
Li2O—LiF—MnO2—Mn2O3
Li1.8Mn1.2O2.93F0.07
FM-3M
1
3.44
1.61
330
Example 18
Li2O—LiF—MnO2—Mn2O3
Li1.8Mn1.2O2.85F0.15
FM-3M
1
3.38
1.63
320
Comparative
LiF—LiMnO2
Li2MnO2F
R3-M
1
3.00
1.50
130
example 1
Comparative
—
LiCoO2
R3-M
1
2.00
2.00
145
example 2
Comparative
LiF—LiCoO2
LiCoO1.95F0.05
R3-M
1
1.95
2.03
143
example 3
Comparative
LiF—LiCoO2
LiCoO1.9F0.1
R3-M
1
1.90
2.05
140
example 4
Table 1 shows that the initial discharge capacity of the batteries of Examples 1 to 18 ranged from 220 to 343 mAh/g.
The initial discharge capacities of the batteries of Examples 1 to 18 were larger than those of Comparative Examples 1 and 2.
The plausible reason for the larger initial discharge capacities in Examples 1 to 18 is that the crystal structure in Examples 1 to 18 contained more Li.
Table 1 also shows that the initial discharge capacity of the battery of Example 2 is smaller than that of Example 1.
The plausible reason for the smaller initial discharge capacity in Example 2 is that the presence of impurities in the synthesis resulted in high resistance.
Table 1 also shows that the initial discharge capacity of the battery of Example 3 is smaller than that of Example 1.
The plausible reason for the smaller initial discharge capacity in Example 3 is that the presence of more fluorine with low electronegativity resulted in lower electron conductivity and poor load characteristics.
Table 1 also shows that the initial discharge capacities of the batteries of Examples 4 to 6 are smaller than that of Example 1.
The plausible reason for the smaller initial discharge capacities in Examples 4 to 6 is that lower Li/Mn ratios in Examples 4 to 6 resulted in less lithium involved in charging and discharging.
Table 1 also shows that the initial discharge capacities of he batteries of Examples 7 and 8 are smaller than that of Example 1.
The plausible reason for the smaller initial discharge capacities in Examples 7 and 8 is that the presence of more fluorine with low electronegativity resulted in lower electron conductivity and poor load characteristics.
Table 1 also shows that the initial discharge capacity of the battery of Example 9 is smaller than that of Example 1.
The plausible reason for the smaller initial discharge capacity in Example 9 is that in addition to an oxidation-reduction reaction of Mn in the charge/discharge process an oxidation-reduction reaction of oxygen is more utilized and thereby destabilizes the crystal structure in the charge process.
Table 1 also shows that the initial discharge capacities of he batteries of Examples 10 and 11 are smaller than that of Example 1.
The plausible reason for the smaller initial discharge capacity in Examples 10 and 11 is that in addition to an oxidation-reduction reaction of Fe or Co in the charge/discharge process an oxidation-reduction reaction of oxygen is more utilized and thereby destabilizes the crystal structure in the charge process.
Table 1 also shows that the initial discharge capacity of the battery of Example 12 is smaller than that of Example 1.
The plausible reason for the smaller initial discharge capacity in Example 12 is that the transition metal species was Ni, and an oxidation-reduction reaction of oxygen was less utilized in the charge/discharge process.
Table 1 also shows that the initial discharge capacities of the batteries of Examples 13 and 14 are smaller than that of Example 1.
The plausible reason for the smaller initial discharge capacity in Examples 13 and 14 is that due to the addition of Ni and Co an oxidation-reduction reaction of oxygen was less utilized in the charge/discharge process.
Table 1 shows that x of more than 2.1 in the composition formula (1) (for example, x=2.18 in Example 3) resulted in an unstable crystal structure in the charging process. This resulted in decreased discharge capacity.
Table 1 also shows that x of less than 1.8 (for example, x=1.79 in Example 6) resulted in a Li/Me ratio of less than 2 and a decreased amount of Li involved in charging and discharging. This resulted in decreased discharge capacity.
The results show that x satisfying 1.89≤x≤2 can result in increased discharge capacity.
Table 1 also shows that the initial discharge capacities of the batteries of Examples 15 and 16 are smaller than that of Example 1.
The plausible reason for this is that an increased amount of Mn in Examples 15 and 16 resulted in limited Li diffusion paths. Furthermore, in Example 15, a decrease in Me valence (Mn valence) possibly resulted in insufficient formation of a Mn-oxygen hybrid orbital and insufficient utilization of an oxidation-reduction reaction of oxygen.
Table 1 also shows that the initial discharge capacity of the battery of Example 16 is larger than that of Example 15.
The plausible reason for this is that the larger amount of oxygen in Example 16 than in Example 15 resulted in the formation of a Mn-oxygen hybrid orbital. Thus, an oxidation-reduction reaction of oxygen could be utilized to increase the capacity.
Examples 17 and 18 were inferior in cycle characteristics to Example 1.
The plausible reason for this is that the amount of fluorine in Examples 17 and 18 was small (that is, β was less than 0.5).
The Li/Me ratio and the O/F ratio were changed from those described in Example 1.
Table 2 lists the precursors for the production of positive-electrode active materials of Examples 19 to 35 and the compositions of the positive-electrode active materials thus synthesized.
Except for these, the positive-electrode active materials of Examples 19 to 35 were synthesized in the same manner as in Example 1.
The precursors of Examples 19 to 35 were weighed at the stoichiometric ratio and were mixed in the same manner as in Example 1.
Coin-type batteries were produced from the positive-electrode active materials of Examples 19 to 35 in the same manner as in Example 1.
The capacities of the coin-type batteries of Examples 19 to 35 were measured in the same manner as in Example 1. Table 2 shows the results.
Table 2 lists “(x+y)/(α+β)”, which indicates deficiency, “{2α+β−(3−y)}/y”, which indicates the Me valence, and “(4y−β)/α”, which indicates the valence of oxygen after change, for each sample.
TABLE 2
Initial
discharge
Space
(x + y)/
{2α + β −
(4y −
capacity
Sample
Precursor
Composition
group
(α + β)
(3 − y)}/y
β)/α
mAh/g
Example 19
LiF—Li2O—Mn2O3—MnO2
Li1.4Mn1.2O2.5F0.5
FM-3M
0.87
3.08
1.72
388
Example 20
LiF—Li2O—Mn2O3—MnO2—LiCoO2
Li1.4MnCo0.2O2.5F0.5
FM-3M
0.87
3.08
1.72
384
Example 21
LiF—Li2O—Mn2O3—MnO2
Li1.35Mn1.25O2.6F0.4
FM-3M
0.87
3.08
1.77
373
Example 22
LiF—Li2O—Mn2O3—MnO2
Li1.4Mn1.2O2.6F0.4
FM-3M
0.87
3.17
1.69
361
Example 23
LiF—Li2O—Mn2O3—MnO2—LiNiO2
Li1.4MnNi0.2O2.5F0.5
FM-3M
0.87
3.08
1.72
355
Example 24
LiF—Li2O—Mn2O3—MnO2
Li1.5MnO2.2F0.8
FM-3M
0.83
3.20
1.45
354
Example 25
LiF—Li2O—Mn2O3—MnO2
Li1.5MnO2F
FM-3M
0.83
3.00
1.50
349
Example 26
LiF—Li2O—Mn2O3—MnO2
Li1.35Mn1.25O2.5F0.5
FM-3M
0.87
3.00
1.80
343
Example 27
LiF—Li2O—Mn2O3—MnO2
Li1.6Mn0.9O2F
FM-3M
0.83
3.22
1.30
334
Example 28
LiF—Li2O—Mn2O3—MnO2
Li1.5MnO2.5F0.5
FM-3M
0.83
3.50
1.40
323
Example 29
LiF—Li2O—Mn2O3—MnO2
Li1.6Mn0.9O1.8F1.2
FM-3M
0.83
3.00
1.33
321
Example 30
LiF—Li2O—Mn2O3—MnO2
Li1.4Mn1.2O2.2F0.8
FM-3M
0.87
2.83
1.82
315
Example 31
LiF—Li2O—Mn2O3—MnO2
Li1.4Mn1.1O2F
FM-3M
0.83
2.82
1.70
291
Example 32
LiF—Li2O—Mn2O3—MnO2
Li1.4Mn1.2O2F
FM-3M
0.87
2.67
1.90
288
Example 33
LiF—Li2O—Mn2O3—MnO2
Li1.4Mn1.2O1.5F1.5
FM-3M
0.87
2.25
2.20
267
Example 34
LIF—Li2O—Mn2O3—MnO2—LiCoO2—LiNiO2
Li1.4(Mn0.54Ni0.13Co0.13)1.5O2.5F0.5
FM-3M
0.87
3.08
1.72
309
Example 35
LIF—Li2O—Mn2O3—MnO2—LiCoO2—LiNiO2
Li1.4Mn0.4Ni0.4Co0.4O2.5F0.5
FM-3M
0.87
3.08
1.72
296
Table 2 shows that the initial discharge capacities of the batteries of Examples 19 to 25 are larger than that of Example 1.
The plausible reason for this is that initially introduced deficiency (that is, “(x+y)/(α+β)” was less than 1) in Examples 19 to 25 resulted in the formation of more Li diffusion paths in Examples 19 to 25 than in Example 1. This probably increased the capacity.
In addition to the introduced deficiency, a smaller change in the valence of oxygen in Example 19 than in Example 1 (or a larger “(4y −β)/α”, which indicates the valence of oxygen after change, in Example 19 than in Example 1) could result in a more stable structure. This probably increased the capacity.
Table 2 also shows that the initial discharge capacity of the battery of Example 20 is smaller than that of Example 19.
The plausible reason for this is that the addition of Co in Example 20 made it difficult to utilize an oxidation-reduction reaction of oxygen in the charge/discharge process when compared with Example 19.
Table 2 also shows that the initial discharge capacity of the battery of Example 21 is smaller than that of Example 19.
The plausible reason for this is that a decreased Li/Mn ratio in Example 21 resulted in a smaller amount of available Li in Example 21 than in Example 19.
Example 21 was inferior in cycle characteristics to Example 1.
The plausible reason for this is that the amount of fluorine in Example 21 was small (that is, β was less than 0.5).
Table 2 also shows that the initial discharge capacity of the battery of Example 22 is smaller than that of Example 19.
The plausible reason for this is that more utilization of an oxidation-reduction reaction of oxygen in Example 22 than in Example 19 destabilized the structure.
Table 2 also shows that the initial discharge capacity of the battery of Example 23 is smaller than that of Example 19.
The plausible reason for this is that the addition of Ni in Example 23 made it difficult to utilize an oxidation-reduction reaction of oxygen in the charge/discharge process when compared with Example 19.
Table 2 also shows that the initial discharge capacity of the battery of Example 24 is smaller than that of Example 19.
The plausible reason for this is that a small “(x+y)/(α+β)”, which indicates initial deficiency, in Example 24 resulted in lower randomness of elements in Example 24 than in Example 19 and unsatisfactory formation of Li diffusion paths. Furthermore, a large change in the valence of oxygen (or a small “(4y−β)/α”, which indicates the valence of oxygen after change) in Example 24 probably destabilized the structure.
Table 2 also shows that the initial discharge capacity of the battery of Example 25 is smaller than that of Example 19.
The plausible reason for this is that a small “(x+y)/(α+β)”, which indicates initial deficiency, in Example 25 resulted in lower randomness of elements in Example 25 than in Example 19 and unsatisfactory formation of Li diffusion paths.
Table 2 also shows that the initial discharge capacity of the battery Example 26 is the same as the initial discharge capacity of the battery of Example 1.
The plausible reason for this is that introduced deficiency and a smaller change in the valence of oxygen in Example 26 stabilized the structure, but a decreased Li/Mn ratio resulted in a decreased amount of available Li.
Table 2 shows that the initial discharge capacities of the batteries of Examples 27 to 29 are smaller than that of Example 1.
The plausible reason for this is that more utilization of an oxidation-reduction reaction of oxygen in Examples 27 to 29 than in Example 1 destabilized the structure. Furthermore, a large change in the valence of oxygen (or a small “(4y−β/α”, which indicates the valence of oxygen after change) in Examples 27 to 29 probably destabilized the structure.
Table 2 shows that the initial discharge capacity of the battery of Example 30 is smaller than that of Example 1.
The plausible reason for this is that introduced deficiency and a smaller change in the valence of oxygen in Example 30 stabilized the structure, but a decreased Li/Mn ratio resulted in a decreased amount of available Li.
Table 2 shows that the initial discharge capacity of the battery of Example 30 is smaller than that of Example 26.
The plausible reason for this is that the Me valence (Mn valence) was smaller in Example 30 than in Example 26 (that is, “{2α+β−(3−y)}/y” was small). More specifically, the smaller Mn valence probably resulted in insufficient formation of a Mn-oxygen hybrid orbital and insufficient utilization of an oxidation-reduction reaction of oxygen.
Table 2 also shows that the initial discharge capacity of the battery of Example 30 is larger than that of Example 32.
The plausible reason for this is that a larger amount of oxygen in Example 30 than in Example 32 probably resulted in increased utilization of an oxidation-reduction reaction of oxygen.
Table 2 shows that the initial discharge capacity of the battery of Example 31 is smaller than that of Example 1.
The plausible reason for this is that introduced deficiency and a smaller change in the valence of oxygen in Example 31 stabilized the structure, but a decreased Li/Mn ratio resulted in a decreased amount of available Li.
Table 2 shows that the initial discharge capacity of the battery of Example 31 is smaller than that of Example 25.
The plausible reason for this is that the Me valence (Mn valence) was smaller in Example 31 than in Example 25 (that is, “{2α+β(3−y)}/y” was small) More specifically, the smaller Mn valence probably resulted in insufficient formation of a Mn-oxygen hybrid orbital and insufficient utilization of an oxidation-reduction reaction of oxygen.
Table 2 shows that the initial discharge capacity of the battery of Example 32 is smaller than that of Example 1.
The plausible reason for this is that introduced deficiency and a smaller change in the valence of oxygen in Example 32 stabilized the structure, but a decreased Li/Mn ratio resulted in a decreased amount of available Li.
Table 2 shows that the initial discharge capacity of the battery f Example 32 is smaller than those of Examples 26 and 30.
The plausible reason for this is that the Me valence (Mn valence) was smaller in Example 32 than in Examples 26 and 30 (that is, “{2α+β−(3−y)}/y” was small). More specifically, the smaller Mn valence probably resulted in insufficient formation of a Mn-oxygen hybrid orbital and insufficient utilization of an oxidation-reduction reaction of oxygen.
Table 2 shows that the initial discharge capacity of the battery of Example 33 is smaller than that of Example 1.
The plausible reason for this is that the Me valence (Mn valence) was smaller in Example 33 than in Example 1 (that is, “{2α+β−(3−y)}/y” was small). More specifically, the smaller Mn valence probably resulted in insufficient formation of a Mn-oxygen hybrid orbital and insufficient utilization of an oxidation-reduction reaction of oxygen.
Table 2 also shows that the initial discharge capacity of the battery Example 33 is smaller than that of Example 32.
The plausible reason for this is that a smaller amount of oxygen in Example 33 than in Example 32 probably resulted in decreased utilization of an oxidation-reduction reaction of oxygen.
Table 2 shows that the initial discharge capacities of the batteries of Examples 34 and 35 are larger than that of Example 13.
The plausible reason for this is that initially introduced deficiency (that is, “(x+y)/(α+β)” was less than 1) in Examples 34 and 35 resulted in the formation of more Li diffusion paths in Examples 34 and 35 than in Example 13. Furthermore, a smaller change in the valence of oxygen in Examples 34 and 35 than in Example 13 could result in a more stable structure. This probably increased the capacity.
Table 2 also shows that the initial discharge capacities of the batteries of Examples 34 and 35 are smaller than that of Example 19.
The plausible reason for this is that the addition of Ni and Co in Examples 34 and 35 made it difficult to utilize an oxidation-reduction reaction of oxygen in the charge/discharge process when compared with Example 19.
A positive-electrode active material according to the present disclosure can be suitable for a positive-electrode active material of batteries, such as secondary batteries.
Nakura, Kensuke, Natsui, Ryuichi
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